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Article

Annealing-Driven Structural and Optical Evolution of Amorphous Ge–C:H Alloys

by
Domenica Marabello
1,2,
Alma Cioci
1,
Mauro Sgroi
1 and
Paola Benzi
1,2,*
1
Dipartimento di Chimica, University of Torino, Via P. Giuria 7, 10125 Torino, Italy
2
Centre for Crystallography (CrisDi), University of Torino, 10124 Torino, Italy
*
Author to whom correspondence should be addressed.
Processes 2025, 13(11), 3457; https://doi.org/10.3390/pr13113457
Submission received: 29 September 2025 / Revised: 20 October 2025 / Accepted: 22 October 2025 / Published: 28 October 2025
(This article belongs to the Section Materials Processes)
Browse Figures
Versions Notes

Abstract

Amorphous hydrogenated germanium–carbon alloys (Ge1−xCx:H) were synthesized by X-ray-activated Chemical Vapor Deposition and investigated to evaluate the effects of annealing on their structure, composition, and properties given the limited information available on their behavior at high temperatures. Thermogravimetric and elemental analyses showed that the materials are stable up to 573 K; above this temperature, the carbon and hydrogen content progressively decrease, favoring structural reorganization. XRPD and Raman analyses demonstrate that the as-deposited films are fully amorphous, while annealing promotes the progressive formation of crystalline Ge. This crystallization occurs heterogeneously through the nucleation of small “islands” embedded within the sample matrix. Optical measurements reveal a narrowing of the band gap with increasing annealing temperature and time. The weak contribution of sp2-carbon observed in some Raman spectra indicates that band gap reduction is mainly governed by the overall composition and the variation of germanium hydrogen bonding configuration, rather than by graphitization. The study also notes that the parameter B1/2 does not follow a regular trend due to the complex nature of the material’s microstructural evolution during annealing. These results provide a comprehensive picture of the annealing-driven transformations in Ge–C:H alloys relevant for the design of thermally stable optoelectronic materials.

1. Introduction

Germanium–carbon alloys (Ge1−xCx:H) have attracted considerable attention for their tunable electrical, optical, and structural properties, which depend strongly on the Ge/C ratio, the deposition parameters and/or the precursors [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51]. Owing to these characteristics, they are promising materials for applications such as multilayer antireflection coatings and protective infrared windows [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43], as well as in electronic and optoelectronic devices [1,2,13,45,46,47,48,49,50].
Several deposition techniques have been employed for their synthesis, including reactive sputtering [1,2,3,4,5,6,7,8,9,10,11,12,28,29,30,31,32,33,34,35,36,37,38,43,44,45,46,47,51] and chemical vapor deposition (CVD) using organogermanium precursors or GeH4/hydrocarbon mixtures [16,17,18,19,20,21,22,23,24,25,26,27,39,40,41,42,50,52]. Although numerous studies have addressed the correlation between microstructure and physical properties—such as optical, electrical, and mechanical characteristics—much less attention has been devoted to their thermal stability and the evolution of composition, morphology, structure, optical parameters, and band gap behavior under annealing, with only a few articles published on this topic [17,41,53,54].
Yet, understanding these processes is crucial for device reliability, since Ge–C–H materials may operate at elevated temperatures or experience localized heating during use.
In previous studies, we synthesized hydrogenated germanium carbides (Ge1−xCx:H) by X-ray-activated chemical vapor deposition (X-CVD) from GeH4/hydrocarbon mixtures, obtaining materials with a broad compositional range and different optical, physical and structural properties [55,56,57,58,59,60,61].
Here, we report the results of a systematic investigation on the combined effects of annealing temperature and duration on the composition, bonding configuration, morphology structure, and optical response of X-CVD Ge1−xCx:H films.
This study addresses this gap by providing a comprehensive analysis of the thermal behavior of these materials in the 373–923 K range. The effect of the annealing time at the same temperature was also investigated.
The main objectives of this work are (1) to determine the compositional and bonding changes induced by thermal treatments; (2) to correlate these changes with the structural and morphological evolution; and (3) to evaluate their impact on the optical parameters and band-gap behavior.
This approach provides detailed information to clarify the mechanisms governing the transformation of Ge–C–H networks under heat treatment and to assess their implications for thermally stable optoelectronic applications.

2. Materials and Methods

Materials. Monogermane (GeH4) was prepared as described in the literature [18], starting from GeO2 and KBH4. Ethyne (C2H2) was supplied by SIAD S.p.A. (Società Italiana Acetilene e Derivati, Bergamo, Italy) at 99.99% stated purity. Both gases were purified by bulb-to-bulb distillation under vacuum and dried with sodium sulfate.
Radiolysis. Hydrogenated germanium-carbide (Ge1−xCx:H) were synthesized using X-ray Activated Chemical Vapor Deposition (X-CVD) from germane and ethyne gaseous mixtures. The precursor mixtures were prepared with ethyne concentrations of 10%, 30%, and 50% (molar ratio) in 365 mL Pyrex vessels at a total pressure of 700 Torr prior the X-ray irradiation, using standard vacuum techniques for reactant handling.
The deposition was performed using a GILARDONI CPXT-320 X-ray tube (maximum output: 320 keV) (Gilardoni S.p.A. Mandello del Lario, (LC), Italy).
The vessels were irradiated for 6 h at a dose rate of 0.5 × 104 Gy/h with 200 keV X-rays; the total adsorbed dose rate was about 3.0 × 104 Gy h−1. During irradiation, the temperature never exceeded 310 K. To avoid any possible oxidation by oxygen in the air, all analyses on the solids were performed soon after opening the vials or the annealing step.
Annealing. The annealing experiments were performed under vacuum in the 373–923 K range, in a tubular Carbolite oven (Fisher Scientific, Loughborough, UK), with the samples placed in a platinum boat within a quartz vial that was subsequently sealed using standard high-vacuum techniques, before being placed in the furnace.
Three series of samples were obtained by annealing the solids derived from the radiolysis of the different GeH4/C2H2 mixtures at different temperatures and annealing times: Ac10, Ac30, and Ac50 from mixtures with 10%, 30%, and 50% of ethyne, respectively. The annealing routines were performed by subjecting each sample to subsequent thermal treatments, increasing the temperature and/or the annealing time. After each step, a small amount of the sample was drawn for analysis, while the remaining sample was annealed in the following step.
Elemental Analysis. The composition of the solid products, both before and after annealing, was determined using a Thermo Electron Corporation CHNS-O analyzer (Aspert, Torino, IT) for the C and Htotal (bonded and unbonded) content, while the Ge content was calculated as the difference. The density was measured with a Berman balance (vs. toluene). The results reported are the mean of at least three independent measurements performed on different aliquots of the same sample.
Thermogravimetric Analysis (TGA). The thermogravimetric analysis was carried out with a TA Q500 model from TA Instruments (New Castle, DE, USA) by heating samples at a rate of 10 °C/min in nitrogen from 50 to 1150 °C.
Gas Chromatography–Mass Spectrometry analysis (GC-MS). Gas species evolved after annealing were analysed by GC-MS on a Varian 3400-Finnigan ITD instrument equipped with an Alltech AT-1 chromatographic column (polydimethylsiloxane, length 30 m, I.D. 0.25 mm, film thickness 1.0 µm) (Alltech/Grace Carnforth, UK). Before injection, the GC oven was cooled at about 213 K introducing liquid nitrogen; afterward the temperature was raised at 373 K at 10 K/min.
Electron ionization was performed at 70 eV and acquisition of ions was achieved in the 15–650 u mass range.
Infrared Spectroscopy (IR). The analyses by IR spectroscopy (KBr pellets) were performed with a FTIR Bruker Equinox 55 instrument (Bruker Italia, Milano, IT) equipped with a program for the deconvolution of overlapped peaks. The resolution was 2 cm−1. All spectra were recorded at room temperature.
Raman Spectroscopy. Raman spectra were acquired on a Renishaw’s Raman spectrophotometer equipped with a microscope and a 633 nm laser exciting source (10 mW laser power, 10 accumulations of 10 s) (Renishaw S.p.A. Pianezza, TO, IT). A Bruker RFS100 spectrophotometer, equipped with a Nd:YAG (neodymium-doped yttrium aluminium garnet) laser as the irradiating source (emitting at 1.064 μm), (Bruker Italia, Milano, IT) was also employed. The average laser powers were within the 50–195 mW range, with typically 15,000 averaged scans.
UV-Visible Spectroscopy (UV-Vis). The UV-Vis spectra were obtained with a Perkin-Elmer Lambda 15 spectrophotometer (Perkin-Elmer, Milano, Italy).
X-Ray Powder Diffraction (XRPD). The X-ray Powder Diffraction patterns were collected at room temperature using the Atlas S2 Rigaku-Oxford Diffraction Gemini R-Ultra diffractometer (Rigaku Europe SE, Neu-Isenburg, Germany), equipped with mirror-monochromatized Cu−Kα (1.5418 Å) radiation. Each powder pattern was collected by rotating the sample by 60 degrees, with an exposure time of 60 s.
Scanning Electron Microscopy (SEM). The SEM images of the samples were acquired using a FEI Quanta 200 3D SEM/FIB (FEI Company Eindhoven, The Netherlands).

3. Results and Discussion

Synthesis via X-ray irradiation of germane/ethyne mixtures (10%, 30%, and 50% ethyne concentration) yielded Ge1−xCx:H as a solid product. These films, deposited on the bottom of the ampoules, were obtained in quantities of approximately 40, 150, and 280 mg, corresponding to the three ethyne concentrations, respectively.

3.1. Thermogravimetric Analysis

To investigate the annealing effect on the obtained materials, they were preliminarily examined by thermogravimetric analysis. The results for the materials obtained with 10%, 30%, and 50% ethyne are shown in Figure 1.
All samples were thermally stable up to 600 K. At this temperature, they underwent a first weight loss, whose extent depended on the composition of the reacting mixture: it varied between 18% and 25% when the ethyne percentage ranged between 10% and 50%. A second weight loss was observed between 900 K and 1050 K and was nearly the same for the solids obtained from all the mixtures (about 47%).
No weight loss was found in the 425–485 K temperature range, as indeed observed in the TGA of solids obtained from the radiolysis of mixtures with CH4, C2H6, or C3H8 [62]. This suggests that when ethyne is used in the irradiated mixture the thermal stability is enhanced.
To explain the absence of this weight loss it must be considered that the solids obtained by radiolysis of GeH4/C2H2 mixtures have a significantly higher carbon [56,57] content than those obtained from mixtures with alkanes. This favors the substitution of some Ge-Ge bonds with stronger Ge-C bonds [63] and the increase in the degree of cross-linkage [59] which leads to a consequent decrease in the number of peripheral chains [62] responsible for the weight loss in this temperature range.
The absence of this weight loss suggests that the sharp increase in the solid carbon content observed when ethyne is used in the irradiated mixture [56,57] enhances the thermal stability.
This is in agreement with the expected replacement of some Ge-Ge bonds by stronger Ge-C bonds [63] and the observed increase in the cross-linkage degree [59], which leads to a consequent decrease in the number of peripheral chains [62] that are responsible for the weight loss in this temperature range.
We have not performed a systematic analysis of the gaseous species released during annealing; however, to gain an indication of the species released, we analyzed (with GC–MS technique) the gases evolved after annealing the sample obtained with 50% ethyne at 673 K. Mixed GexCy hydrides, with 1 to 2 Ge atoms and 1 to 4 C atoms, were detected. Hydrocarbons with 1 to 3 C atoms were also found.
Thermogravimetric analyses (TGA) were carried out under flowing nitrogen with a continuous heating rate of 10 K/min, primarily to determine the onset of thermal degradation and the temperatures corresponding to major mass losses. These reference temperatures then guided the design of subsequent isothermal annealing experiments, which were performed under vacuum to enable detailed characterization of composition, morphological, structural, and optical modifications.

3.2. Elemental Analysis and Density

The effects of annealing on the characteristics and properties of the materials, synthesized using 10%, 30%, and 50% ethyne concentration in the precursor mixture, were further investigated with respect to both the temperature (Ta) and time (ta) of the thermal treatment.
The full set of samples derived from the three starting materials, including both the as-deposited and the thermally treated specimens, are categorized into the Ac10, Ac30, and Ac50 series, respectively.
The Ge1−xCx:H samples was investigated through a sequential cumulative annealing process. The annealing procedure was applied independently to each un-annealed sample across the sequential target temperatures. For the Ac30 and Ac50 series (and the Ac10 series at 573 K), the protocol involved an initial annealing for 1.0 h at Ta, followed by the removal of a portion for intermediate characterization; the remaining material was then subjected to a subsequent 1.5 h of re-annealing at the same Ta, resulting in a final cumulative time of 2.5 h at that step.
For the Ac10 series at Ta = 673, 873, and 973 K, the material was annealed directly for a total duration of 2.5 h in a single step, with analysis performed only at the end of the treatment. Crucially, the material used for annealing at a given Ta was the unanalysed residual portion of the sample that had been subjected to all previous, lower-temperature treatments.
Table 1 reports the empirical formulas of the materials obtained from the GeH4/C2H2 mixtures with 10% (Ac10 series), 30% (Ac30 series), and 50% (Ac50 series) ethyne, both before and after each annealing treatment.
In Figure 2 and Figure 3, the carbon molar fraction (C/Ge + C) and the hydrogen content for the Ac10, Ac30, and Ac50 series of samples are reported as a function of the annealing temperature.
Confirming the TGA results, no composition variation is observed when the Ta value is lower than 573 K. For temperatures higher than 573 K, Table 1 and Figure 2 and Figure 3 show that the annealing-induced weight losses are accompanied by changes in the solids’ composition. In fact, both the C and H solid contents decrease relative to Ge as Ta increases. Moreover, it is interesting to note that both values show a regular decreasing trend as Ta increases.
An annealing time effect also emerges from the results shown in Figure 2 and Figure 3. In fact, variations in both the carbon molar fraction and hydrogen content are observed for solids annealed at the same temperature for different times. This effect is more evident at lower Ta and explains the difference observed when comparing the TGA and Table 1 data: the first weight loss starts at about 600 K (TGA), whereas a significant carbon content decrease is already evident at 573 K (Table 1). This finding can be attributed to the different annealing times. In the TGA experiments, the temperature increases from room temperature to 600 K in about 30 min (with a heating rate of 10 K/min), while the first annealing results in Table 1 were obtained after a one-hour heating at 573 K.
In Figure 4, the density values for the Ac10, Ac30, and Ac50 series samples are reported as a function of their carbon and hydrogen content, both before and after annealing at different temperatures. A regular trend is observed with respect to both components.
As the annealing temperature increases, a variation in density is also observed. For the un-annealed samples, the density ranges between about 2.0 and 1.67 when the ethyne percentage in the irradiated gaseous mixture varies from 10% to 50%. However, for the annealed materials, the value increases with the annealing temperature. As an example, for the solid obtained with 50% ethyne, the density ranges from 2.0 at Ta = 573 K to 2.25 at Ta = 923 K.

3.3. Infrared and Raman Spectroscopy

Infrared and Raman spectroscopy were used to investigate the annealing-induced modifications of the materials’ structures and hydrogen bonding configurations.
Figure 5 shows the IR spectra of the solids obtained from the irradiation of mixtures with 10%, 30%, and 50% of C2H2 before thermal treatment.
In the 2700–3050 cm−1 range and between 1900–2150 cm−1, all the solids exhibit overlapped bands attributable to the stretching vibrations of hydrogen atoms in CHn (n = 1, 2) and GeHn (n = 1–3) groups, respectively [1,4,7,21,27,59,64,65,66,67].
In the region between 1200 and 1500 cm−1, the bond bending signals of CHn groups are typically found [1,4,7,21,27,59,64,65,66,67], whereas between 400 and 1200 cm−1, the signals are an envelope of overlapped broad bands attributable to the stretching, bending, wagging, and rocking vibrations of different bond systems such as Ge-H, Ge-C, CH2, and Ge-C-C [1,4,7,21,59,64,65,66,67].
It is worth noting that the ratio between the integrated signal I is defined by:
I = ∫ α ( ω ) ω / d ω
where α(ω) is the absorption coefficient at wavenumber ω), for the GeHn (n = 1–3) species with respect to that of CHn (n = 1–3) decreases as the ethyne percentage in the irradiated mixture increases. This indicates that some Ge-H bonds are substituted by Ge-C bonds.
In Figure 6 are reported the IR spectra, in the range 1750–3050 cm−1, of the solid obtained with 50% of C2H2, both before and after the thermal treatment at different temperatures and annealing times.
In agreement with the results reported above, no variations in the line shape and peak intensity are observed in the IR spectra if Ta is lower than 573 K, confirming the thermal stability of the materials below this temperature.
When the sample is annealed at 573 K or higher, our results are partially consistent with those reported for a-GeC:H obtained by RF sputtering [12], which show a two-step hydrogen evolution process with the breaking of Ge-H bonds occurring first, followed by the breaking of C-H bonds. This process occurs because of the higher C-H bond energy (413.4 kJ/mol) compared to that of Ge-H (349.8 kJ/mol) [68].
In fact, although a modification of the IR spectra is observed in both the Ge-H and C-H stretching zones starting from Ta = 573 K, the breakage of Ge-H bonds appears favored, as indicated by the decrease in the νGe-H/νC-H intensity signal ratio (Figure 6) observed up to 673 K.
At this temperature, the νGe-H signals have fully disappeared, while the signals of the C-Hn groups remain clearly visible, disappearing completely only after annealing at 823 K for 2.5 h.
Furthermore, it should be noted that, as a consequence of the thermal treatment, a shift in the GeHn (n = 1–3) bond stretching band toward a lower frequency is observed, due to substructure modification. The deconvolution of the broad band between 1900–2150 cm−1 reveals three signals at approximately 1990, 2025, and 2045 cm−1 for the un-annealed sample and the sample annealed at 573 K for 1 h, and two signals, at approximately 1990 and 2025 cm−1, after annealing at the same Ta for two hours. The attribution of the single peaks is not possible, because their position can be influenced by the presence of C atoms bonded to hydrogenated Ge atoms [57], but it is clear that the thermal treatment causes not only a decrease in all the signals but also a change in their relative intensity. Considering the results of the elemental analysis and the possible attribution of the deconvoluted signals [57], it is reasonable to suppose that the annealing first leads to a decrease in the number of more hydrogenated groups, and then to their disappearance.
Moreover, the observed decrease in the intensity of the CHn group signals is probably not so much due to the rupture of C-H bonds, but rather to the decrease in the solid carbon content, which is lost as hydrogenated fragments (both pure and mixed) as suggested by GC-MS analysis results. This hypothesis is further supported by the elemental analysis results, which show a clear decrease in the C/Ge atomic ratio and hydrogen content with increasing annealing temperature.
In Figure 6, the deconvolution of this signal is shown for the un-annealed and annealed (at Ta = 673 K and ta = 2.5 h) Ac50 series samples. The un-annealed sample and the sample annealed at 573 K exhibit three deconvoluted peaks, attributable to in-phase and out-of-phase vibrations of hydrogen atoms in the CH2 groups (around 2860 and 2930 cm−1), and to the CH stretching mode (around 2893 cm−1). Two other peaks around 2872 and 2955 cm−1, attributable to stretching modes of the CH3 groups [1,4,7,21,64,65,66,67,69,70], are observed after annealing at a higher Ta. It is also interesting to note that up to 573 K, the intensity of the integrated absorption band of the deconvoluted CH signal is greater than that of CH2. The value of the νC-H/νC-H2 intensity signal ratio is 4.4, 1.65 and 1.2 for the un-annealed and annealed at 473 K for 1 and 2.5 h, respectively.
For higher temperatures, the CH3 signals appear, even though the hydrogen content drastically decreases. The data suggests that the hydrogen atoms become highly mobile upon heating, resulting in the progressive evolution toward more hydrogenated C–Hn groups.
Further consideration can be given by examining the band at 590 cm−1, which is attributable both to Ge-C stretching and the superimposed GeHn wagging mode [59]. Crucially, the intensity of this band does not significantly decrease in the annealed samples, in contrast to the GeHn stretching signals, which sharply decrease and subsequently disappear. This observation supports the hypothesis that Ge-H bonds are replaced by Ge-C bonds upon annealing.
An annealing time effect is also evident in Figure 6. Variations in peak’s intensity are clearly visible, depending not only on the temperature but also on the annealing time (at the same Ta value), which indicates that these transformations occur slowly.
To obtain further information about the germanium structures or to distinguish between the two hybridizations (sp2 or sp3) of the carbon atoms in the material matrix or in the carbon-segregated zones inside the alloy, Raman spectroscopy was used.
In amorphous germanium (a-Ge), four main peaks are expected at 80, 160, 220, and 275 cm−1, corresponding to transverse acoustic (TA)-like, longitudinal acoustic (LA)-like, longitudinal optic (LO)-like, and transverse optic (TO)-like modes, respectively. For germanium in crystalline form (c-Ge), a narrow signal at 300 cm−1 is generally found [8,20,71,72,73,74,75,76].
The sp2-bonded carbon clusters show two broad signals around 1580 cm−1 and 1350 cm−1, attributable to the graphite G and D lines, respectively. However, sometimes only a single signal, the so-called amorphous C band, is present in the 1400–1500 cm−1 region [6,8,20,52,71,77,78,79,80].
The sp3-hybridized C atoms are observed as a very wide band at approximately 1100 cm−1 only when a UV laser (244 nm) is used [77,78].
In Figure 7a,b, the germanium and amorphous carbon regions of the Raman spectra for the sample obtained with 50% ethyne, before and after the 2.5 h annealing at different temperatures, are shown.
No signal attributable to c-Ge, a-Ge, or sp2-hybridized carbon is present for the un-annealed sample, and no variations in the line shape and peak intensity are observed if Ta is lower than 573 K. At this temperature, the signals for the a-Ge (TA and TO peaks) become visible.
After annealing at 673 K, a broad peak between 210 and 330 cm−1 is clearly visible. This peak is attributable to the presence of both amorphous and crystalline germanium phases embedded in the solid matrix. Deconvolution of this peak reveals three signals around 270, 280 and 300 cm−1. The first, with a full width at half maximum (FWHM) of ~45 cm−1, can be attributed to the TO mode of the a-Ge. The other two signals originate from the crystalline phase. The signal at 302 cm−1 (FWHM ~10 cm−1) is related to relatively small crystallites, while the signal at 285 cm−1 (FWHM ~25 cm−1) is related to the larger ones. This indicates a poly-dispersed system where the FWHM and peak position depend on the crystalline size [76,81]. Germanium crystallinity increases with Ta, and after annealing at Ta = 923 K, the amorphous Ge fraction becomes undetectable, leaving only the signal of the crystalline phase. Deconvolution of this signal yields two peaks at 300 cm−1 (FWHM ~6 cm−1) and 290 cm−1 (FWHM ~14 cm−1). Additionally, a narrowing and shift in the deconvoluted peaks (the shift being more evident for the peak at the lower wavenumber) are observed, likely due to the increase in crystal size [76].
A semi-quantitative measure of the crystalline volume fraction in the germanium zones can be obtained from the integrated intensities of the bands related to the disordered and crystalline phases. This is achieved by considering the ratio RC = (ISC + ILC)/(ISC + ILC + IA), where ISC and ILC are the intensities of the small- and large-sized crystalline phases, respectively, and IA is the intensity of the amorphous peak in the Raman spectra [82].
The RC value is approximately 50% at Ta = 673 K and becomes 100% at Ta = 923 K. These temperatures are significantly lower than those observed by Y.X. Jie et al., who reported an RC of approximately 60% at Ta = 948 K and full crystallization only after the annealing at 1073 K or higher [83].
However, it is important to consider that our samples contain a high amount of Ge-H bonds. All these bonds are broken between 573 K and 673 K (as evidenced by the IR spectra), which may favor the rearrangement of germanium atoms. This process can lead to “islands” of Ge atoms embedded in the material matrix. These Ge atom groupings evolve from an amorphous to a crystalline phase through a process enhanced by the annealing temperature, which favors the diffusion of Ge atoms or clusters. In this transformation, the smaller crystals can be considered an intermediate state between the amorphous and the crystalline phases.
In the Raman region, where bands of the sp2-hybridized carbon are expected (1400–1500 cm−1), no signal is found until Ta = 673 K (Figure 7b). The Raman band observed near 1450 cm−1 is assigned to the CH2 and CH3 deformation (δ(CH2) and δas(CH3)) modes. The low-frequency region exhibits two distinct features at approximately 1100 cm−1 and 1000 cm−1, which correspond to skeletal C-C stretching (ν(C-C)) vibrations, representing different chain conformations.
At 673 K, a weak band appears in the zone between the D and G signals of graphitic carbide, likely due to amorphous carbon [6,8,20,52,71,77,78,79,80]. At higher temperature (823 K), two very weak signals appear at 1330 and 1565 cm−1. The peak at the higher wavenumber can be associated with the G band, which is the only allowed E2g vibrational mode of crystalline graphite. It is caused by the bond stretching of all pairs of sp2 atoms in both rings and chains. The peak at 1330 cm−1 is attributable to the D band, which is due to the breathing modes of sp2 atoms in rings. This signal is often attributed to disorder-activated phonon modes, which become Raman active, due to the lack of long-range order in amorphous graphitic materials [80,84].
The ID/IG ratio of the deconvoluted signals increases with Ta, from about 0.3 to about 0.8 as Ta increases from 673 K to 823 K. This suggests an increase in the number or size of ordered aromatic rings in the sample [85]. On the other hand, the very low intensity of the D and G bands suggests a very limited conversion from the sp3 to the sp2 configuration for the carbon atoms.
The Raman spectra of the un-annealed samples obtained with 10% and 30% of ethyne show analogous characteristics. As with the solid obtained with 50% ethyne, the Raman spectra of the annealed samples show no significant changes until the annealing temperature rose to 573 K. After the annealing at 673 K, Ge crystallization is observed for both samples, and at Ta = 923 K, the amorphous Ge fraction disappears.
However, differences are observed in the carbon zone (1400–1500 cm−1) of solids obtained with 10% of ethyne. No signal for sp2-hybridized carbon is found, even after annealing at 823 K. This is probably due to the lower initial C/Ge atomic ratio (which is 1.60), allowing for better Ge and C mixing. This reduces the probability of C-C bond formation during the bond rearrangement process that follows the annealing-induced desorption of hydrogen and small fragments, thus preventing the formation of sp2 carbon zones.
The elemental analysis and the results of Raman and IR spectroscopies indicate that the reactions involving Ge-H bond breaking are favoured and, due to the annealing, CHn (n = 1, 3), C-CHn (n = 1, 2), and/or Ge-Hn (n = 1–3) bonds are partially (or totally) broken and replaced by Ge-C and/or Ge-Ge bonds. Meanwhile, the hydrogen content and C/Ge atomic ratio in the solids decrease.
This finding aligns with the lower bond energy of Ge-H (≤321 kJ/mol) compared to Ge-C (439.9 kJ/mol) and C-H bonds (413.4 kJ/mol). Additionally, although in Ge2 clusters the Ge-Ge bond energy (259 kJ/mol) is lower than the Ge-H bond energy, it becomes significantly higher in clusters with more than two Ge atoms (for example, it is approximately 666.8 kJ/mol in Ge3 clusters) [86].

3.4. X-Ray Powder Diffraction

To investigate possible annealing-induced modifications to the materials’ structure, the XRPD patterns of both the un-annealed and annealed samples obtained with 50% of ethyne were collected and are shown in Figure 8.
The XRPD pattern of the un-annealed sample exhibits no diffraction peaks, confirming the absence of crystalline phases. After annealing at 573 K, weak and broad reflections appear at 2θ ≈ 27° and 50°, and additional peaks at 45.3° and 53.4° emerge after annealing at 673 and 923 K. These reflections can be assigned to crystalline germanium, corresponding to the Ge(111), Ge(220), and Ge(311) planes [87,88]. With increasing temperature, the diffraction peaks gradually sharpen, indicating progressive crystal growth. However, their residual broadening shows that the germanium phase remains composed of very small size crystallites rather than large bulk crystals.
The approximate crystal size was calculated using the Scherrer Equation, yielding values of 1.71, 4.27, and 7.98 nm for the samples annealed at 573 K, 673 K, and 923 K, respectively.
We calculated the approximate crystal size, D, using the Scherrer Equation [89]:
D = Kλ/(Bcosθ)
where λ is the X-ray wavelength (0.15418 nm for Cu Kα radiation), B is the Full Width at Half Maximum (FWHM) in radians, θ is the Bragg angle (in degrees), and K is the Scherrer constant. The calculated crystal sizes were 1.71, 4.27, and 7.98 nm for the samples annealed at 573 K, 673 K, and 923 K, respectively.
These XRPD observations are fully consistent with Raman spectroscopy. Both techniques confirm the absence of crystalline Ge in the un-annealed sample and reveal its progressive crystallization upon annealing. The process is heterogeneous, proceeding through the nucleation of small Ge crystalline “islands” dispersed within the sample matrix. These little crystallites coexist with amorphous regions and increase in number and size with rising annealing temperature, as reflected by the sharpening of diffraction peaks and the concurrent evolution of Raman features.

3.5. Scanning Electron Microscopy

The SEM micrographs of the samples obtained with 50% ethyne, annealed at different temperatures, are shown in Figure 9a–d.
Figure 9a shows the image of the side in contact with the substrate surface of the sample annealed at 573 K. Figure 9b–d show the images of the surface along the growth direction of the samples annealed at 573 K, 673 K, and 773 K (for 2.5 h), respectively.
As can be seen from the images, the morphology of the side in contact with the substrate is almost featureless, while the other side of all the samples is characterized by the presence of spherical agglomerates of various sizes.
These results suggest that the formation of the material obtained by the X-ray irradiation occurs through an island deposition mechanism. The first stages of material formation happen in the gas phase. In fact, radiolysis causes the formation of ions and, more importantly, radicals that react with the molecules present through a radical propagation mechanism. The formed nuclei are deposited on the substrate’s surface and can agglomerate due to diffusion, generating islands that then grow by coalescence, leading to the formation of a complete layer. Furthermore, the post-deposition irradiation that the solids undergo soon after deposition causes bonds in the material network to break and induces a grafting phenomenon. This phenomenon contributes to the continuous growth of the solid, favoring the formation of compact layers.
Moreover, as Figure 9 shows, the layers on the top surface of the deposited film are not complete, and the annealing clearly increases the coalescence process. In fact, comparing the images, a significant change in the surface morphology can be observed as the Ta value increases, indicating that an aggregation process between neighbouring particles occurs, leading to coalescence. This phenomenon increases with temperature.

3.6. Optical Properties

The optical band gap (Eopt), defined as the energy difference between the extended states of the valence and conduction bands, is a key characteristic of amorphous semiconductors. The Tauc procedure is commonly used to determine Eopt from UV–Vis absorption spectra [90,91]. According to this method, a linear extrapolation of the (αhν)1/2 versus photon energy (hν) plot to the x-axis provides the Eopt value. This relationship is expressed as:
(αhν)1/2 = B1/2(hν − Eopt)
where α is the absorption coefficient, h is the Planck’s constant, ν is the photon frequency, and B1/2 (the Tauc slope) is a parameter related to the structural disorder of the amorphous network. A lower B1/2 value indicates a higher degree of disorder [92,93].
Table 2 reports the Eopt and B1/2 values for the samples obtained with 10%, 30%, and 50% ethyne, both before and after annealing at different Ta and ta values.
As shown in Table 2, the Eopt values of the Ac10, Ac30, and Ac50 series samples decrease with increasing annealing temperature and time. In hydrogenated amorphous carbides or binary carbides, band gap narrowing is often attributed to the change in the hybridization of the carbon atoms from sp3 to sp2 [28].
However, in our case, even though signals of sp2-hybridized carbon appear in the Raman spectra of the Ac30 and Ac50 series samples annealed above 673 K, their intensity is too low to account for the observed band gap reduction.
Moreover, a decrease in Eopt is also observed for the same series at lower annealing temperatures, where sp2-carbon signals are absent, and in Ac10 series, where such signals are not detected at any temperature.
In hydrogenated binary amorphous semiconductors, band gap narrowing may also result from changes in elemental composition. Moreover, the concentration of hydrogen bonded to germanium is another factor influencing Eopt and in our previous work, [86] we showed that more hydrogenated Ge groups correlate with larger band gaps.
In Figure 10a,b, the Eopt values for the samples obtained with 10%, 30%, and 50% ethyne, before and after annealing at different Ta and ta, are reported as a function of the carbon molar fraction and hydrogen content. A regular trend is observed, suggesting a close relationship between annealing-induced compositional changes and the evolution of Eopt.
It can therefore be hypothesized that the decrease in Eopt primarily arises from the concurrent reduction in C and H content, consistent with previous observations in amorphous Ge-C alloys [44,47]. The decrease in more hydrogenated GeHn groups up to 573 K, and their complete disappearance at higher annealing temperatures (as shown in the IR spectra), further contributes to the decrease in the Eopt value.
In hydrogenated amorphous carbides or binary carbides, the band gap narrowing is often accompanied by a decrease in the B1/2 value and a corresponding increase in sp2 carbon content (higher disorder). However, as shown in Table 2, the B1/2 value in our samples does not follow a regular trend with increasing annealing temperature and time. Considering the Raman results (above discussed) the observed irregularity in our data suggests that the structural disorder is not solely governed by the shift in carbon hybridization. Instead, as indicated by our Raman and XRPD analyses, the annealing process promotes the formation and coexistence of heterogeneous domains (specifically crystalline and amorphous germanium and emerging graphitic regions) embedded within the material matrix. It is therefore reasonable to conclude that the anomalous evolution of B1/2 is primarily influenced by the presence and growth of these heterogeneous domains which evolve in a complex and irregular manner during annealing, rather than a simple sp3 to sp2 transition.

4. Conclusions

This study provides novel insights into the thermal evolution of Ge–C–H materials, revealing how compositional, bonding, and structural transformations are interrelated and strongly influence optical properties. Our findings establish that these materials exhibit robust thermal stability up to 573 K. Above this threshold, a cascade of compositional and structural changes is triggered. The compositional changes increase material density, modify the bonding configuration, and induce structural rearrangements.
A key finding is the identification of Ge–H bond dissociation as a precursor to Ge crystallization, which begins as early as 673 K, whereas C–H bonds show greater thermal stability, persisting up to 823 K. This differential behavior enables selective tuning of bonding environments through thermal treatment. Annealing also alters the Ge–H and C–H bonding configuration, leading to the progressive reduction and eventual disappearance of the more hydrogenated GeHn groups, concurrent with the formation of increasingly hydrogenated CHn groups.
The work also demonstrates that carbon distribution within the Ge matrix critically affects phase segregation and sp2-carbon formation, with low-carbon samples showing no graphitization due to more homogeneous mixing. Such control over carbon clustering and Ge crystallinity is essential for tailoring material properties for optoelectronic applications, including photodetectors, infrared sensors, and tunable semiconducting layers.
Importantly, the study establishes clear correlations between annealing conditions, carbon and hydrogen content, bond configurations (e.g., GeHn and CHn groups), and optical band gap, offering a roadmap for band-gap engineering in Ge–C–H systems. The absence of a clear trend in the B1/2 parameter further highlights the complex interplay between structural disorder and phase coexistence, suggesting that fine control over processing conditions is necessary to optimize performance.
Looking forward, future research could explore in situ monitoring of crystallization dynamics, controlled doping strategies, or integration of these materials into device architectures to fully exploit their tunable properties. These findings thus provide a solid foundation for the rational design of Ge–C–H materials in advanced electronic and optoelectronic technologies.

Author Contributions

Investigation, D.M., M.S. and A.C.; Methodology, D.M., M.S. and P.B.; Validation, D.M., A.C. and P.B.; Writing—original draft, D.M., A.C., P.B. and M.S.; Writing—review & editing, D.M., P.B., A.C. and M.S. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support from MUR (Ministero dell’Università e della Ricerca).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. TGA results of the materials obtained with 10%, 30%, and 50% ethyne.
Figure 1. TGA results of the materials obtained with 10%, 30%, and 50% ethyne.
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Figure 2. Carbon molar fraction of the Ac10, Ac30, and Ac50 series samples as a function of annealing temperature.
Figure 2. Carbon molar fraction of the Ac10, Ac30, and Ac50 series samples as a function of annealing temperature.
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Figure 3. Hydrogen content of the Ac10, Ac30, and Ac50 series samples as a function of annealing temperature.
Figure 3. Hydrogen content of the Ac10, Ac30, and Ac50 series samples as a function of annealing temperature.
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Figure 4. Density values of the un-annealed and annealed solids (for 1 and 2.5 h) obtained from the different GeH4/C2H2 mixtures as a function of their carbon and hydrogen content.
Figure 4. Density values of the un-annealed and annealed solids (for 1 and 2.5 h) obtained from the different GeH4/C2H2 mixtures as a function of their carbon and hydrogen content.
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Figure 5. Infrared spectra of the solid obtained from irradiation of mixtures with 10%, 30%, and 50% of C2H2 before thermal treatment.
Figure 5. Infrared spectra of the solid obtained from irradiation of mixtures with 10%, 30%, and 50% of C2H2 before thermal treatment.
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Figure 6. IR spectra of the solid obtained from irradiation of mixture with 50% of C2H2, before and after thermal treatment, at different temperatures and annealing times in the 1750–3050 cm−1 range. Note: spectra scale is not the same.
Figure 6. IR spectra of the solid obtained from irradiation of mixture with 50% of C2H2, before and after thermal treatment, at different temperatures and annealing times in the 1750–3050 cm−1 range. Note: spectra scale is not the same.
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Figure 7. Germanium (a) and amorphous carbon (b) regions of the Raman spectra of the sample obtained with 50% ethyne, before and after annealing (2.5 h) at different temperatures.
Figure 7. Germanium (a) and amorphous carbon (b) regions of the Raman spectra of the sample obtained with 50% ethyne, before and after annealing (2.5 h) at different temperatures.
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Figure 8. XRPD patterns of the sample obtained with 50% ethyne before and after annealing at different temperatures.
Figure 8. XRPD patterns of the sample obtained with 50% ethyne before and after annealing at different temperatures.
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Figure 9. SEM micrographs of the samples obtained with 50% ethyne, annealed at 573 K (a,b), 673 K (c), and 773 K (d).
Figure 9. SEM micrographs of the samples obtained with 50% ethyne, annealed at 573 K (a,b), 673 K (c), and 773 K (d).
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Figure 10. Eopt values of the Ac10, Ac30, and Ac50 series samples as a function of carbon molar fraction (a) and hydrogen content (b).
Figure 10. Eopt values of the Ac10, Ac30, and Ac50 series samples as a function of carbon molar fraction (a) and hydrogen content (b).
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Table 1. Empirical formulas of the samples obtained from the GeH4/C2H2 mixtures with 10%, 30%, and 50% ethyne, before and after each annealing treatment.
Table 1. Empirical formulas of the samples obtained from the GeH4/C2H2 mixtures with 10%, 30%, and 50% ethyne, before and after each annealing treatment.
Gas Mixture
C2H2 Percentage		Series Sample NameTemperature (K)
and Time (Hours)		Solid Products Empirical Formula a
not annealedGeC1.60H4.50
573—1 hGeC1.01H2.55
10%Ac10573—2.5 hGeC0.85H2.12
673—2.5 hGeC0.61H1.43
823—2.5 hGeC0.42H0.30
923—2.5 hGeC0.40H0.29
not annealedGeC2.65H6.38
573—1 hGeC1.73H4.08
573—2.5 hGeC1.50H3.20
30%Ac30673—1 hGeC1.20H2.78
673—2.5 hGeC1.05H2.28
823—1 hGeC0.84H0.62
823—2.5 hGeC0.54H0.35
923—2.5 hGeC0.50H0.22
not annealedGeC3.10H7.05
573—1 hGeC1.93H4.79
573—2.5 hGeC1.80H4.10
50%Ac50673—1 hGeC1.20H2.70
673—2.5 hGeC0.89H2.10
823—1 hGeC0.62H0.60
823—2.5 hGeC0.60H0.43
923—2.5 hGeC0.45H0.12
a determinations are affected by an error within 10%.
Table 2. Eopt and B1/2 values of the Ac10, Ac30, and Ac50 series samples.
Table 2. Eopt and B1/2 values of the Ac10, Ac30, and Ac50 series samples.
C2H2 PercentageTemperature (K)
and Time (Hours)		EoptB1/2
not annealed2.00110.4
573—1 h1.3157.7
10%573—2.5 h1.1689.9
673—2.5 h0.94130.0
823—2.5 h0.66107.5
923—2.5 h
not annealed2.8973.0
573—1 h2.4649.7
573—2.5 h2.2073.8
30%673—1 h1.5082.7
673—2.5 h0.8864.6
823—1 h 50.5
823—2.5 h0.87
not annealed3.5107.0
573—1 h2.791.0
573—2.5 h2.373.0
50%673—1 h1.1189.2
673—2.5 h1.0270.0
823—1 h0.7660.0
823—2.5 h 44.3
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Marabello, D.; Cioci, A.; Sgroi, M.; Benzi, P. Annealing-Driven Structural and Optical Evolution of Amorphous Ge–C:H Alloys. Processes 2025, 13, 3457. https://doi.org/10.3390/pr13113457

AMA Style

Marabello D, Cioci A, Sgroi M, Benzi P. Annealing-Driven Structural and Optical Evolution of Amorphous Ge–C:H Alloys. Processes. 2025; 13(11):3457. https://doi.org/10.3390/pr13113457

Chicago/Turabian Style

Marabello, Domenica, Alma Cioci, Mauro Sgroi, and Paola Benzi. 2025. "Annealing-Driven Structural and Optical Evolution of Amorphous Ge–C:H Alloys" Processes 13, no. 11: 3457. https://doi.org/10.3390/pr13113457

APA Style

Marabello, D., Cioci, A., Sgroi, M., & Benzi, P. (2025). Annealing-Driven Structural and Optical Evolution of Amorphous Ge–C:H Alloys. Processes, 13(11), 3457. https://doi.org/10.3390/pr13113457

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